Patch: back-passwd needs pwent mutex (ITS#1794)

Full_Name: Hallvard B. Furuseth
Version: HEAD (2002-05-03)
OS: Linux
URL: http://folk.uio.no/hbf/OpenLDAP/back-passwd.txt
Submission from: (NULL) (158.36.148.34)
back-passwd uses getpwent() and setpwfile(), which use static data.
It needs a mutex to make sure these operations can complete without
interference from another back-passwd call. Here is a patch.